Rabbit cytochrome P-450 isozymes 2 and 5 are present in lung and liver. In the liver, but not the lung, the concentrations of these isozymes are increased by treatment of rabbits with phenobarbital. Homologues of isozymes 2 and 5 have been detected in lungs of mice, rats, hamsters, guinea pigs and monkeys. Although homologues of isozyme 2 are also present in livers of these species, hepatic homologues of isozyme 5 are not detected in any species except the hamster. Treatment of hamsters with phenobarbital does not increase the concentration of the isozyme 5 homologue in liver. In microsomal preparations from the Clara cell, alveolar type II cell, and alveolar macrophage fractions isolated from rabbit lung, the relative concentrations (% total P-450) of cytochrome P-450 isozyme 2 and 5 are similar to those determined with the microsomal fraction from intact lung. The absolute concentrations of these isozymes are somewhat (about 2-fold) higher in the Clara cell preparation than in preparations from type II cells or intact lung and substantially higher than the concentrations found with the macrophage. In contrast, the highest concentrations of isozyme 6 are found in microsomes prepared from intact lung. Analysis of cDNA probes for rabbit liver and lung isozyme 2 indicates that microheterogenous (greater than 95% homology) forms of this isozyme exist. Results of restriction mapping show a difference in the probes at a position corresponding to amino acid 174. The derived sequence for the hepatic isozyme is not identical to the sequence that has been published for the protein. This difference may be due to the animal populations used in the separate studies or to microheterogeneity in liver.